Grain refining of metals and alloys

ABSTRACT

An innoculating additive comprised of a nickel carrier and tungsten oxide particles is useful in grain refining alloys such as nickel-base superalloys.

Davies et al.

GRAIN REFINING OF METALS AND ALLOYS Inventors: Peter Wesley Davies; JohnPhilip Dennison, both of Swansea, England Assignee: The InternationalNickel Company,

Inc., New York, N.Y.

Filed: May 21, 1973 Appl. No.: 362,521

References Cited UNITED STATES PATENTS 12/1917 Ladoff 75/93 G 8/1927l/1940 1/1940 7/1971 l/l972 10/1973 51 Dec. 30, 1975 De Bats 75/93 GMansfield 75/138 Mansfield 75/138 X Lambert et al. 75/171 X Davies eta1. 75/135 Metz 75/129 X Primary ExaminerAl1en B. Curtis AssistantExaminer-Thomas A. Waltz Attorney, Agent, or Firm-Raymond J. Kenny; EwanC. MacQueen ABSTRACT An innoculating additive comprised of a nickelcarrier and tungsten oxide particles is useful in grain refining alloyssuch as nickel-base superalloys.

11 Claims, N0 Drawings GRAIN REFINI NG OF METALS AND ALLOYS The subjectinvention is addressed to the grain refinement of metals and alloys,particularly nickel-base superalloys, during the process ofsolidification from the molten state. i

As the metallurgist is aware, reduction in the grain size of a metal oralloy can be effected if solid particles which act as nuclei are addedto a molten bath of the metal to be cast. To that end, in our UnitedKingdom patent specification 1,239,066 we advanced the concept ofincorporating in a molten metal matrix a grain refining nucleatingadditive consisting of particles of metal oxide dispersed in a carriermetal, the additive having been formed by internal oxidation of analloy. Upon being introduced into'the melt the metal carrier dissolves,the oxide particles being left as a fine dispersion in the melt matrix.We specifically proposed the use of innoculants consisting of internallyoxidized copper-nickel, copper-cobalt, copper-nickel-cobalt, copper-ironand copper-iron-nickel alloys, the nickel, cobalt and iron oxidizingpreferentially to the copper and acting as the grain refining nuclei.

We further deemed it most preferable that nucleating oxide particles beless than 1 micron nn) in size, though particles up to two microns mightbe used. This followed from the fact that in general the smaller theoxide particle size the greater is the degree of grain refining in themelt. This, however, desirable though it be, gives rise to attendantdifficulties for it is both difficult and expensive to prepare additivescontaining, for example, nickel oxide particles smaller than 1 pm. Too,the presence of copper in some alloys is undesirable, if not actuallydetrimental. Furthermore, subsequent investigation reflected that thegrain refining effect of the copper-containing additives was not aspronounced in respect of thicker section castings.

In. any case, it has now been found unexpectedly that if instead ofnickel, cobalt or iron the preferentially oxidized metal is tungsten andthat nickel as opposed to copper is the carrier metal, then the degreeof grain refinement obtained by this special combination is notdependent to the same extent on oxide particle size. This obviates theneed for oxide particles of very small size. And, of course, recourse toa copper carrier is eliminated. It might be mentioned that copper isseldom, if ever, an intentionally added constituent in nickelbasesuperalloys.

Furthermore, it was also found that when using tungsten oxide particlesin very slowly cooled ingots, substantially better grain refinement wasexperienced than by the use of any other tested oxide particles. Thislends markedly to producing thick section castings. Indeed, anintegrally precision cast nickel-base superalloy turbine wheel, i.e.,blade and hub, has been prepared using tungsten oxide as the innoculanton a nickel strip carrier and the grain size in the hub was actually asfine as that in the blade. Insofar as we are aware, this has never beenaccomplished heretofore, at least by the incorporation of a nucleatingagent.

Accordingly, the present invention provides a process for grain refininga metal or alloy during solidification from the molten state byintroducing to a melt of the metal or alloy at least a small buteffective amount of a grain refining additive comprised of a nickelcarrier and tungsten oxide particles preferably formed by internaloxidation, so that the nickel dissolves and solid tungsten oxide nucleiare introduced into the melt, the

oxide particles not being restricted to sizes of less than 1 pm as apractical matter. A particle size of up to at least 2 pm can be usedquite satisfactorily.

To form the additive by internal oxidation a nickeltungsten alloy mustbe used. While a wide range of nickel-tungsten alloys can be utilized,the size of the oxide particles formed is dependent on the alloycomposition. Thus, as the tungsten content of the alloys increases, therate of internal oxidation decreases and the size of the oxide particlesformed tends to become larger. in this connection, it might be said thatthe larger a given size does mean a smaller ratio of oxide particles tonickel carrier strip and this could add to cost. For such reasons thetungsten content of the alloy should not exceed 17% and moreadvantageously should not exceed 6%. On the other hand, should thetungsten content be too low, the number of particles formed by oxidizinga given weight of the alloy decreases, so that an excessive amount ofthe carrier needs to be introduced. It is therefore preferred that thealloy contains at least 1% of tungsten and more preferably at least 3%.

The nickel-tungsten alloy should preferably be single-phased since in atwo-phased alloy the second phase may enter the melt as undissolvedparticles which, while not necessarily harmful, could nonethelessinterfere with the nucleation of grains. The efficiency of nucleation isfound to fall off as the temperature of the melt rises, and in treatingnickel-based alloys particularly the temperature of the melt preferablyshould not exceed about 1500C. and more beneficially should not exceedabout 1470C.

The internal oxidation of the alloy to obtain the oxide nuclei is mostconveniently effected by heating the alloy in air, although otheratmospheres such as carbon dioxide can be used. It is preferable, inorder to obtain as many oxide nuclei as possible, to-carry out theoxidation process for a period sufficient such that the alloy isinternally oxidized virtually right through the center. However, toavoid excessive scaling losses, the heating should be stopped once thedesired oxidation has been achieved. We have found that scaling lossesare reduced if the oxidation process in air is continued to the pointthat an external article coating is formed and the remainder of theoxidation process is conducted in a substantial vacuum. Thisparticularly applies to nickeltungsten alloys containing less than about10% tungsten. The alloy may become oxidized externally as well asinternally. It is of advantage that surplus oxide be removed as bybrushing or pickling, before the additive is introduced into the melt.The additive may also advantageously be degreased before use.

To ensure that the nuclei become uniformly dispersed throughout themelt, it is important to prevent local chilling of the melt when theadditive is introduced. For this reason, it is advantageous to useadditives which have a small cross section, and it is particularlyadvantageous to use strip or foil from 0.05 to 0.4 mm. in thickness.Another reason for using such foil is that the depth of penetration ofthe internal oxidation is a function of time and temperature, so that analloy of greater thickness requires a longer time at the oxidizingtemperature if it is to be fully internally oxidized. With a fullyinternally oxidized alloy, as little as 0.1% additive by weight of themelt would introduce sufficient nuclei to be effective in refining thegrains, but in general from about 0.2 or 0.3 to 0.5 or 0.8% of additiveby weight of the melt is a suitable addition.

Additives containing tungsten oxide particles are particularly useful inthe grain refining of a wide range of melts of alloys containing nickel,such as alloys having as their base nickel and copper or nickel andchromium or nickel, chromium and cobalt, in each case with or withoutiron up to 50%. Such alloys include those which have a base composed offrom 20 to 80% nickel, to 35% chromium, 0 to 40% cobalt and 0 to 50%iron, and which may also contain one or more of the elements such asaluminum or titanium up to each, and molybdenum, niobium, tantalum,tungsten, vanadium, zirconium, boron and hafnium in the amounts commonlypresent in high temperature alloys, e.g., up to molybdenum, up to 8% ofniobium and/or tantalum, up to 25% tungsten, up to 3% vanadium, up to 2%zirconium, up to 0.5 or 1% boron (preferably less than 0.1% boron), andup to 5% hafnium.

The additive may also be useful for treating melts which are refined andcast at lower temperatures, for example, melts of copper-aluminumalloys, stainless steel and other alloy steels, and brasses, e.g., 70%copper-30% zinc brass.

The following data is given as generally illustrative of the invention.

Three nickel-tungsten alloys containing 2%, 4%, and 15% tungsten,respectively, in the form of strip 0.12 mm. thick were degreased andinternally oxidized by heating in air. The Ni-2%W and Ni-4%W alloys wereheated at 1140C. and the Ni-l5%W alloy at 1200C. In each case theheating was continued for four hours, but with the 2% and the 4%tungsten alloys the air was removed after minutes and the heatingcompleted in vacuum and this minimized scaling losses. The vacuumtechnique was unsuitable for the Ni-15%W alloy since the internaloxidation proceeded extremely slowly. The predominant composition of theadditives thus prepared was Ni-3% tungsten oxide for the additive madefrom the alloy containing 2% tungsten, Ni-5% tungsten oxide for theadditive made from the alloy containing 4% tungsten and Ni-19% tungstenoxide for the additive made from the alloy containing 15% tungsten. Theaverage size of the oxide particles was 0.2 pm, 0.4 pm and 1.5 pm,respectively.

The oxidized strip was quenched into water, and surface oxide filmremoved by rubbing with emery paper, or by scratch brushing, preferablyafter coldrolling slightly to fracture the scale, or by pickling, forexample, in hydrochloric acid.

Melts of a nickel-base superalloy (No. 1) containing, by weight, 0.09%carbon, 0.46% silicon, 0.54% iron,

An additive in the form of the strip prepared as described above andattached to a silica stirrer was introduced into melts of this alloy.The melts were poured into sythetic mullite molds (2.8 inches indiameter, 7.5 inches high, 1 inch wall thickness) preheated to 800 to1000C. and a hot-topping compound was then added, the melts beingallowed to solidify.

The extent of grain refinement was determined by comparing the as-caststructure of a transverse section of the inoculated ingots with that ofa similar section taken from an untreated ingot otherwise prepared inthe same fashion. The sections were taken from a position mid-waybetween the bottom of the shrinkage pipe and the bottom of the ingotavoiding columnar crystals grown from the bottom.

The results are set forth in Table 1.

TABLE I Tungsten Oxide The following was used to classify the ingotstructures:

E medium equiaxial grain: 2.5 to 4.5 mm. diameter C,, medium columnargrain; 2.5 to 4.5 mm. diameter E coarse equiaxial grain; 4.5 to 6.0 mm.diameter C coarse columnar grain; 4.5 to 6.0 mm. diameter C very coarsecolumnar grain; mm. diameter It can be seen that grain refinementoccured in all instances in which the nickel-tungsten oxide additives ofvarying oxide particle size were employed. It would appear that theextent of grain refinement is substantially independent of oxideparticle size since the particle size in ingot No. 4 was 1.5 um and theresults were quite satisfactory.

To illustrate that grain refinement can be obtained with the aid oftungsten oxide-containing additives over a wide range of temperatures ofthe melt, further tests were carried out in which Ni-5% tungsten oxideadditive was added to melts of alloys having the compositions shown inTable 11.

TABLE ll COMPOSITION (Wt.

Alloy No. C Si Fe Mn Mg Cr Ti Al Ni Co Mo Nb B Zr 2 0.07 1.0 1.0 1.00.012 19.5 2.35 1.40 bal. 2.0 0.3 0.0025 0.06 3 0.13 1.0 1.0 1.0 15.01.2 4.7 bal. 20.0 5.0 0.006 0.10 4 0.05 0.35 bal. 0.35 19.0 1.0 0.6 531.0 3.0 5.3 0.0025 5 0.05 0.25 1.0 0.1 20.0 2.3 0.4 bal. 14.0 4.5 5.00.0025 0.03

19.2% chromium, 2.46% titanium, 1.5% aluminum and 16.4% cobalt, thebalance being essentially nickel, were prepared in a high frequencyinduction furnace.

The results are reported in Table III which show that significant grainrefinement was achieved in each case:

TABLE III Tungsten Oxide Pouring Proportion lngot Alloy Amount in AmountTemp. lngot of each No. No. Addmve(%) Added(- (C.) Structure grain(%) 52 None 1445 C,, 100 6 2 S 0.3 1445 E 100 7 3 None 1425 C, 100 8 3 5 0.31425 C,, E 20-80 9 4 None 1450 C,, 100 10 4 5 0.3 1450 c,,, E, 40-60 1 l5 None 1395 C 100 12 5 5 0.3 1395 E 100 As indicated herein, a turbinewheel was integrally 15 resorted to without departing from the spiritand scope precision cast using the subject nickel-tungsten oxidenucleating additive. In this connection, the alloy used to produce thewheel was of the nickel-base superalloy type and was of the followingapproximate composition: 0.05%C, 11.75%Cr, 4.5%Mo, 6%Al, 0.7%Ti 2%Nb,0.01%B, 0.1%Zr, balance nickel. The alloy was vacuum melted and cast ininvestment shell molds preheated to 800C. with a metal temperature of1460C. The wheel was cut in half to show the grain size in the hub aswell as the blade. The grain size was uniform throughout both sections,being less than about 1/16 inch in the hub. A similar wheel was castwithout aid of the innoculant and the grain size in the hub was'approximately A to inch, approximately 4 to 8 times larger. It has beenconsidered that as a consequence of the fact that the blades and hubsoperate under different loads and are exposed to different elevatedtemperatures, it would be of considerable benefit if the grain size inthe hub region could be reduced, the thought being that a fine grain wasmore important in the hub than in the blade section.

In addition to the above information and data, the subject invention hasbeen used to grain refine a 25%Cr-20%Ni stainless steel and an 18%Nimaraging steel. It is considered that copper base alloys can be grainrefined. Superalloys containing 50% or more of nickel, from 6% to 30%chromium, up to 30% iron, aluminum in an amount of at least 0.4 or 0.5%to 8%, up to 6% titanium, e.g., 0.5 to 5% titanium, up to 8%, e.g., l to6%, molybdenum, up to 20%, e.g., 1 to 5%, tungsten, up to 8%, e.g., 1 to6%, niobium, up to 8%, e.g., 1 to 6%, tantalum, together with amounts ofvanadium, boron, zirconium and hafnium given herein, are generallyrepresentative of other compositions deemed responsive to the invention.

As reflected by the above description, the process of the invention canbe used both in the casting of ingots and in the production of castings,i.e., articles and parts made by casting. The castings may be made byinvestment casting in vacuum, air or an inert atmosphere, or byconventional sand casting methods.

While the oxide formed by internal oxidations of nickel-tungsten alloysis referred to in this specification as tungsten oxide, it is suspectedthat in practice the oxidation of such alloys may lead to additivescontaining some amount of nickel oxide. In-other words, a mixed oxide,for example, NiO W0 may be dispersed in the nickel carrier. Irrespectiveof this, the actual tungsten oxide content should preferably be from 1or 2 to 8%.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be of the invention, as those skilled in the art willreadily understand. Such modifications and variations are considered tobe within the purview and scope of the invention and appended claims.

We claim:

l. A process for grain refining a metal or alloy during solidificationfrom the molten state which comprises introducing into a melt of themetal or alloy an in- I noculating additive comprised of a nickelcarrier and tungsten oxide particles in at least a small but effectiveamount sufficient to induce grain refinement upon solidification, nickeldissolving in the melt and solid tungsten oxide nuclei being dispersedtherein.

2. A process according to claim 1 in which the additive is in the formof strip from 0.05 to 0.4 mm. thick.

3. A process according to claim 1 in which the additive comprises aninternally oxidized single-phase alloy.

4. A process according to claim 3 in which the additive comprises aninternally oxidized nickel-tungsten alloy containing 1 to 17% tungsten.

5. A process according to claim 1 in which the alloy contains from 1% to6% tungsten.

6. A process according to claim 1 in which the melt is an alloy with abase of nickel or nickel and copper or nickel and chromium or nickel,chromium and cobalt, in each case with or without iron up to 50%.

7. A process according to claim 1 in which the melt is an alloy having acomposition falling within the following ranges: 20 to nickel, 5 to 35%chromium, 0 to 40% cobalt and 0 to 50% iron, up to 10% aluminum, up to10% titanium, up to 15% molybdenum, up to 8% niobium, up to 8% tantalum,up to 25% tungsten, up to 3% vanadium, up to 2% zirconium, up to 1%boron and up to 5% hafnium.

8. A process according to claim 1 in which the melt is an alloy having acomposition falling within the following ranges: from 6% to 30%chromium, up to 30% iron, aluminum in an amount of at least 0.4 to 8%,up to 6% titanium, up to 8% molybdenum, up to 20% tungsten, up to 0.25%carbon, upto 6% niobium, up to 6% tantalum, up to 3% vanadium, up to0.5% boron, up to 2% zirconium, up to 5% hafnium, and at least about 50%nickel.

9. A process according to claim 1 in which from 0.2 to 0.8% additive byweight of the melt is introduced.

10. A process according to claim 1 in which from 0.2 to 0.5% additive byweight of the melt in introduced.

11. A process in accordance with claim 1 in which the tungsten oxide wasformed by heating in air to the point that an external coating wasformed and the remainder of the oxidation process was conducted in asubstantial vacuum whereby scaling losses are reduced. It

1. A PROCESS FOR GRAIN REFINING A METAL OR ALLOY DURING SOLIDIFICATIONFROM THE MOLTEN STATE WHICH COMPRISES INTRODUCTING INTO A MELT OF THEMETAL OR ALLOY AN INNOCULATING ADDITIVE COMPRISED OF A NICKEL CARRIERAND TUNGSTEN OXIDE PARTICLES IN AT LEAST A SMALL BUT EFFECTIVE AMOUNTSUFFICIENT TO INDUCE GRAIN REFINEMENT UPON SOLIDIFICATION, NICKELDISSOLVING IN THE MELT AND SOLID TUNGSTEN OXIDE NUCLEI BEING DISPERSEDTHEREIN.
 2. A process according to claim 1 in which the additive is inthe form of strip from 0.05 to 0.4 mm. thick.
 3. A process according toclaim 1 in which the additive comprises an internally oxidizedsingle-phase alloy.
 4. A process according to claim 3 in which theadditive comprises an internally oxidized nickel-tungsten alloycontaining 1 to 17% tungsten.
 5. A process according to claim 1 in whichthe alloy contains from 1% to 6% tungsten.
 6. A process according toclaim 1 in which the melt is an alloy with a base of nickel or nickeland copper or nickel and chromium or nickel, chromium and cobalt, ineach case with or without iron up to 50%.
 7. A process according toclaim 1 in which the melt is an alloY having a composition fallingwithin the following ranges: 20 to 80% nickel, 5 to 35% chromium, 0 to40% cobalt and 0 to 50% iron, up to 10% aluminum, up to 10% titanium, upto 15% molybdenum, up to 8% niobium, up to 8% tantalum, up to 25%tungsten, up to 3% vanadium, up to 2% zirconium, up to 1% boron and upto 5% hafnium.
 8. A process according to claim 1 in which the melt is analloy having a composition falling within the following ranges: from 6%to 30% chromium, up to 30% iron, aluminum in an amount of at least 0.4to 8%, up to 6% titanium, up to 8% molybdenum, up to 20% tungsten, up to0.25% carbon, up to 6% niobium, up to 6% tantalum, up to 3% vanadium, upto 0.5% boron, up to 2% zirconium, up to 5% hafnium, and at least about50% nickel.
 9. A process according to claim 1 in which from 0.2 to 0.8%additive by weight of the melt is introduced.
 10. A process according toclaim 1 in which from 0.2 to 0.5% additive by weight of the melt inintroduced.
 11. A process in accordance with claim 1 in which thetungsten oxide was formed by heating in air to the point that anexternal coating was formed and the remainder of the oxidation processwas conducted in a substantial vacuum whereby scaling losses arereduced.